The invention relates to an optical lens, in particular for use as a spectacle lens, comprising a first lens element and at least one second lens element, wherein the first lens element and the second lens element at least partly act together in an achromatic fashion.
The invention furthermore relates to method for producing an optical lens, in particular for use as a spectacle lens.
Such an optical lens is known from the document GB 487 546 A. The optical lens described therein is used as a cataract lens. However, in principle, this lens can also be used as a spectacle lens.
It is generally known that if the spectacle lens is manufactured from only one lens element, spectacle lenses cause chromatic aberration as a result of the wavelength dependence of the refractive indices of the optical materials from which they are manufactured. The chromatic aberrations include the longitudinal colour aberration (also referred to as axial chromatic aberration or longitudinal chromatic aberration), which generates different foci for different wavelengths of the light. In addition to longitudinal colour aberration, there is also, as a further chromatic aberration, the transverse colour aberration (also referred to as colour magnification error or transverse chromatic aberration), which is expressed by coloured fringes or coloured edges in the image plane, which is the retina of the eye in the case of a spectacle lens; this is perceived by the spectacles wearer and considered an annoyance above a certain intensity.
In the case of spectacle lenses, chromatic aberrations, in particular the transverse chromatic aberration, will not be noticeable in an annoying fashion to the spectacles wearer in the case of spectacle lenses with a low power; however, the chromatic aberrations, particularly the transverse chromatic aberrations, increase in spectacle lenses with increasing optical power, independently of whether the defective eyesight to be corrected is based on myopia or hyperopia.
These days, highly-refractive materials, in particular plastics or highly-refractive types of glass, are often used to keep the spectacle-lens thickness as thin as possible for cosmetic reasons. However, precisely materials with a high refractive index have a significantly stronger transverse chromatic aberration because, in general, an increasing refractive index goes hand-in-hand with a lower Abbe number.
Thus, it is desirable at least to reduce such chromatic aberrations, in particular the transverse chromatic aberration, produced by a spectacle lens.
The field of objectives, e.g. for cameras, has disclosed the practice of correcting chromatic aberrations by so-called achromats. In optics, an achromat is understood to mean a system consisting of at least two lenses that consist of materials with different Abbe numbers and/or different refractive indices and hence differ in the dispersion behaviour. Of the two lenses, one is a positive lens, usually manufactured from a material with a higher Abbe number (e.g. crown glass), and the other lens is a negative lens made of a material with a lower Abbe number and hence greater dispersion than the positive lens, with this second lens for example being manufactured from flint glass.
The two lenses are shaped and interconnected on mutually complementary surfaces such that the chromatic aberration is compensated to the best possible extent for two wavelengths. The two lenses then act together achromatically.
Within the meaning of the present invention “at least partly” acting together “in an achromatic fashion” is understood to mean that the achromatic aberration or aberrations need not necessarily be eliminated completely, but is/are at least reduced.
The above-described conventional achromats are not suitable for use as spectacle lenses. Namely, since these achromats are assembled from two complete lenses, they also have a corresponding thickness and, going hand-in-hand with this, an excessive weight.
The lens disclosed in the document GB 487 546A mentioned at the outset consists of two lens elements that substantially have the same refractive index, of which the one lens element is manufactured from flint glass with a refractive index of approximately 1.61 and a reciprocal relative dispersion of approximately 36. The other lens element is manufactured from barium crown glass with a refractive index of approximately 1.61 and a reciprocal relative dispersion of approximately 50. The first-mentioned lens element is a negative lens element, and the lens element mentioned second is a positive lens element. The two lens elements are interconnected on mutually complementary surfaces.
The lens produced thus has a rear side, i.e. a side facing the eye of the wearer, that is entirely formed by the negative lens element, while the front side of the lens, i.e. the side facing away from the eye of the wearer, is partly formed by the surface of the positive lens and, in the margin region thereof, by the surface of the negative lens.
The lens is furthermore afflicted by the disadvantage that it consists of two complete lens elements and hence it is relatively thick and quite heavy.
The professional article “Hybrid diffractive-refractive achromatic spectacle lenses”, W. N. Charman, Opthal. Physiol. Opt. 1994, volume 14, pages 389 to 392 also considers the reduction of chromatic aberrations in spectacle lenses. It emphasizes that achromats that have a lens with a low refractive index and a high Abbe number and a lens with a high refractive index and a low Abbe number, of which the one lens is negative and the other is positive, are not practical as spectacle lenses because they are contrary to the desire of having spectacle lenses with a low thickness and a low weight. In order to resolve the disadvantages of achromats, it proposes to combine a refractive lens with a diffractive element, wherein the combination of the refractive lens and the diffractive element can substantially have the same thickness and the same weight as the refractive lens on its own.
However, a spectacle lens composed of a refractive lens and a diffractive element is very complex in terms of the production thereof since the diffractive element has to be produced with great precision in order to avoid that the diffractive element induces other aberrations.